Illumination systems for projection devices are known per se. They are used, inter alia, in projection devices such as beamers and projection televisions. In such an image projection system, the light generated in the illumination system impinges on an image creation unit, for example, a Liquid Crystal Display (further also indicated as LCD) or, for example, a Digital Light Processing unit (further also indicated as DLP) or, for example, a Liquid Crystal on Silicon (further also indicated as LCoS) after which the image is projected on to a screen or wall. The illumination systems for projection devices typically emit light in a sequence of colors of light which is repeated over time. Each of the subsequent colors of light is used in the projection device to generate a partial image. The image is generated by subsequently superposing the partial images on the screen. The quality of such an image projection device is often indicated by the brightness of the image which can be produced.
One way of emitting the sequence of colors of light by the known illumination system is by using a color wheel. The color wheel is generally arranged between a light source of the illumination system and a light output window of the illumination system. The color wheel comprises a plurality of color segments which determine the sequence of colors of the light emitted by the illumination system by sequentially positioning color segments from the plurality of color segments between the light source and the light output window.
A drawback when using the color wheel having a plurality of color segments is that the color of the light emitted by the known illumination system constantly changes during a so called spoke time. The spoke time is a time interval during which a boundary between two adjacent color segments passes between the light source and the light output window. The constant changing of the color of the light during the spoke time causes undesired color effects in the image produced by the projection device.
One way of overcoming constant changing of the color during the spoke time is to simply switch off the light source during a spoke time. However, switching off the light during the spoke time reduces the brightness of the illumination system which is very undesirable for projection devices. So, alternative illumination systems have been developed to optimally use the light which is emitted during the spoke time. For example, the US patent application US2007/0035703 provides a system for actively compensating for the spoke light. The cited US patent application improves the technique known as spoke light recovery (SLR) which may be employed to use light generated during spoke times. The known illumination system comprises a light source configured to generate a first light level during a non-spoke time of a color wheel. The system further comprises a photodiode assembly configured to measure the first light level to generate a non-spoke light level. A processor is configured to set a spoke light compensation value based on the non-spoke light level. The light generated during the spoke time for a particular pixel is employed if the shade of that particular pixel includes a red, green and blue light level that are each above a threshold. Further, to facilitate smoother transition between non-SLR and SLR and vice versa, the video unit may be configured to subtract some portion of the light generated during the non-spoke times to compensate for the additional light output during the spoke times. This compensation factor is referred to as the spoke time compensation value.
A disadvantage of the known system is that it requires considerable calculating power to use part of the light emitted during the spoke time.